Healthy individuals maintain equilibrium automatically. A sudden loss of postural equilibrium due to vestibular dysfunction is debilitating, since even the simplest movement requires a conscious effort. Over 2 million people who complain of dizziness and disequilibrium seek medical assistance every year in the U.S. Vestibular dysfunction is a diagnostic challenge. Stable vertical posture is critically dependent on normal vestibular function. Vestibular, somatosensory and visual inputs signal excessive deviations from equilibrium which is then restored by muscle contractions. Small, continuous movements of the head in space accompanying this error-correcting process can be measured directly and accurately in three dimensions using rotation sensors worn on a headband. We hypothesize that vestibular deficits are reflected in the temporal and spectral features of head and body movements during postural instability. Even simple measures of head movement intensity during quiet sitting and standing on a solid surface can discriminate normal from abnormal vestibular function. These head stability (HS) measures are also practical for monitoring early recovery of equilibrium after labyrinthine surgery and for predicting outcome of visual rehabilitation with headborne magnifying aids. To give the HS tests superior diagnostic utility, we propose to identify the dynamical features of head and body movements that are most sensitive to vestibular deficits. The ultimate goal of this research is to develop head stability measurements into a sophisticated and easily applied tests of vestibular function. Spectral analysis and nonlinear models and will be used to quantify the effects of well-defined vestibular, visual and other neurological deficits on dynamics of head and body movement in equilibrium. Movement will be recorded in three dimensions sitting, standing and other postures under different combinations of: visual input, support surface compliance, joint immobilization, and externally applied perturbations to the head or trunk. The specific objectives are: (1) To improve sensitivity of the HS test to vestibular sensory deficits by adding test conditions with distorted somatosensation and visions; (2) To derive and validate models of movement dynamics in order to provide a basic understanding of system dynamics and to identify critical parameters reflecting integrity of vestibular inputs; (3) To further refine diagnostic performance of HS tests by using critical parameters derived from movement spectra and from models fitted to movement records: (4) To systematically evaluated the diagnostic performance of the HS and conventional diagnostic tests by sing the relative operating characteristics (ROC) analysis. This approach will result in vestibular diagnostic tests based on measurements of involuntary head and body movements during natural postural activity. Because of their simplicity, the tests have the potential to become a readily, inexpensive tool for clinical diagnosis.